1. Field of the Invention
The present invention relates to a valve assembly and in particular to an assembly of the type in which regulation of the flow of fluid through the valve is achieved by means of translational and/or rotational movement of a shaft to seal or un-seal a valve opening in response to an applied force from an associated force generator.
2. Description of the Prior Art
In a valve assembly having a mechanically journaled shaft friction in the bearing can cause problems. One of the most common is that frictional forces often cause the shaft to stick in a particular position so that a change in the force applied to the shaft in order to move it to another position can have unpredictable results. Moreover, the magnitude of the frictional force itself is often unpredictable, since it may change with the age of the valve, the pressure of the shaft against the bearing or with movement of the shaft within the bearing in a direction not associated with the sealing or un-sealing of the valve opening, so that the force from the force generator which must be applied to the shaft in order to overcome the frictional force can be unpredictable.
In the remainder of the this application a force generator of the solenoid type will be described in more detail in connection with the valve assembly of the present invention. Those skilled in the art will appreciate that the valve assemblies in the description may be modified to apply to other assemblies that employ different force generators acting on the shaft so as to effect the required movement without departing from the invention.
It is known from U.S. Pat. No. 5,787,924 to provide a valve assembly in which a shaft is movable along the coil axis of a solenoid coil type force generator. A controller is provided which operates to vary the current through the coil to vary an electromagnetic force exerted on the shaft and thereby cause its translation to a desired position. The controller then operates to impose an additional oscillatory current to exert an additional, oscillatory force of a predetermined maximum magnitude on the shaft and cause a sympathetic oscillation of the shaft about the desired position by amounts sufficiently small so as not to adversely effect the flow control operation of the valve. This continuous oscillatory motion is designed to prevent the shaft sticking in the mechanical journal arrangement. The maximum amplitude of this additional force may even be made to be dependent, in a known manner, on the magnitude of an error signal representing the known position of the shaft and the desired position.